1. Field of the Invention
The present invention relates to a wafer polishing apparatus for polishing wafers or the like using chemical mechanical polishing (CMP).
2. Description of the Related Art
The polishing of a wafer using the CMP is performed by pressing the wafer against a rotating polishing pad at a predetermined pressure while rotating the wafer, and supplying a mechanochemical abrasive between the polishing pad and the wafer. At this time, the circumference of the wafer is surrounded by a retainer ring, and the backside thereof is held by a carrier and pressed against the polishing pad.
If a mechanism is used that mechanically provides pressure to a back plate placed on the back of the wafer W to press the wafer W, the mechanism tends to apply pressure to only a certain part of the back plate. The use of an air cylinder as the mechanism is a typical example. When pressure is applied to a certain part of the back plate, the back plate is locally deformed and the deformation affects the wafer W, so that the adverse effect on the accuracy of polishing is concerned.
Therefore, a conventional wafer polishing apparatus has a constitution wherein, as shown in FIG. 4 for example, a back plate 4 is pressed through an air bag 1 in a head body 2 so that the wafer W is evenly pressed. In another conventional wafer polishing apparatus shown in FIG. 5, the air bag 1 directly presses the wafer W. These constitutions are effective for evenly pressing the wafer W.
In FIGS. 4 and 5, the wafer W is polished with a polishing pad 20 attached on the upper surface of a polishing table 12. The wafer W is held by a retainer ring 3 at the circumference thereof.
However, the constitutions shown in FIGS. 4 and 5 have a problem that the control of the pressure applied on the wafer W (pressure in the air bag 1) is difficult. That is, the polishing pressure (pressure applied on the wafer W) required by the CMP was conventionally 40 to 60 kPa. However, in recent years, lower polishing pressure setting (e.g., 10 to 20 kPa) has been demanded in order to minimize the unevenness of film thickness on the surface of the wafer.
Controllers of polishing pressure, electro-pneumatic regulators which especially being used often in the CMP polishing apparatus generally have a maximum pressure of 100 kPa or more, and when such controllers are used in a pressure range between 10 and 20 kPa, the accuracy of control may often lower. Therefore, there has been a problem that required polishing quality cannot be easily obtained.
Also in FIGS. 4 and 5, the retainer ring 3 is pressed against the polishing pad 20 through the head body 2; however, if the pressure applied on the retainer ring 3 is excessively high, the control of polishing the edge portion of the wafer W becomes unstable, and a problem that even polishing is interfered arises. It is therefore preferable that the pressure applied on the retainer ring 3 is appropriately controlled.